Hi Tarfu,
As other have said, if your water heater has gas/electric heating capabilities, your water heater may have another switch outside, behind the water heater cover.
Attached is a picture of my Suburban water heater with the outside cover removed. Notice a small black On/Off switch (white lettering) in the lower left corner of the picture. This switch came covered with a silver tape
(see if you or anyone else can see the tape in this picture!) which made finding the switch more... interesting, to say the least.
This switch is to protect the heating element from burning and it should be off anytime there is no water in the water heater (flushing the water heater tank, etc.) Once the water heater is full of water, one needs to turn this switch on for the heating element to work.
My trailer has also a Gas or Electric option switch inside the trailer.
Good luck and tell us how it went!